This invention relates to combining two well-known and proven technologies in a new and unique way to achieve reduced risk to the public by significantly improving the containment of nuclear fission products from accident events in which fuel integrity is compromised; and, by increasing reactor full power days through increased fuel enrichment. These technologies are tri-isotropic (TRISO) fuel, used in high temperature gas-cooled reactors and zircaloy fuel tubes, used in fuel assemblies for light water reactors.
The invention describes a new and unique light water reactor (LWR) nuclear fuel pellet configuration formed using tri-isotropic (TRISO) fuel particles uniformly distributed in a non-fuel bearing metal, metal alloy or ceramic matrix. This new TRISO LWR fuel pellet would replace the uranium oxide pellet in the LWR fuel tube in the fuel assemblies currently used in nuclear power plants. Use of the TRISO LWR fuel pellet would potentially extend the operating cycles of LWRs, reduce the number of fuel assembles replaced in each refueling, reduce the quantity of spent fuel discharged from each reactor, lower operating costs, reduce radioactive waste and reduce or eliminate fuel failure risk and cost. The TRISO fuel and the LWR cladding provide two separate, very robust, barriers against the release of radioactive material to the public thereby reducing the radiological risk in the event of a reactor accident and enhancing public safety.
The TRISO fuel particle has a long history of use in high temperature gas-cooled reactors applications, but is uniquely used in this invention. The particle is a multilayer sphere having layers of protecting material surrounding a central kernel of enriched uranium. The kernel is typically uranium oxide or uranium carbide, having enrichment in U235 that varies depending on the intended use. The kernel is surrounded by layers of carbon, pyrocarbon and silicon carbide. These layers provide a level of structural integrity that has been demonstrated to contain fission products within the particle, precluding release in normal operations and most accident events. By incorporating the TRISO LWR fuel pellet in the LWR fuel rod, a second, and substantial, barrier is formed to preclude the release of fission product material.
To be used in LWR power plants, the TRISO fuel particles must be held in a metal, metal alloy or ceramic matrix that is non-fuel bearing and that has the same external dimensions as the LWR uranium oxide fuel pellet that it would replace. This allows the TRISO LWR fuel pellets to be substituted for the LWR fuel pellets. Once formed, the TRISO LWR fuel pellets are stacked within the zircaloy fuel rods and the fuel rods are combined into assemblies common to LWR power plants.
Suitable materials for the matrix can include stable, heat conducting materials such as zirconium, aluminum, zinc or nickel, and their alloys, or graphite or other ceramic that is compatible with the TRISO particle.
Use of the same LWR fuel rod and fuel assembly configurations allow use of the new TRISO LWR fuel in operating plants without any change to the physical configuration of the reactor vessel or core internals. Consequently, the use of this invention does not anticipate any changes to the current designs of LWR fuel rods or fuel assemblies. Varying inside diameters of the range of LWR fuel rods in current use are accommodated by conforming the diameter of the TRISO LWR fuel pellet to the inside diameter of the LWR fuel rod.
Numerous references apply to the design, fabrication and use of TRISO fuel microspheres. TRISO fuel has a history dating back to the 1970's when it was first considered for use in advanced design reactors including the high temperature gas cooled reactor. References can also be found for the use of TRISO fuel in pebble bed reactors. This invention uses the same TRISO fuel as an element of the TRISO LWR fuel pellet's unique design, but would not require revision, modification or change to any fabrication strategy, except as it may apply to enrichment.
Similarly, numerous references apply to LWR fuel pellet design, fabrication and use. The intent of this invention is to apply dimensions of existing LWR fuel pellet designs to the new fuel pellet. The new TRISO-LWR pellet is markedly different in all other respects from the existing LWR fuel pellets currently in use.